1. Field of the Invention
This invention relates generally to imaging systems that can capture multiple images of the same object simultaneously, for example images of different spectral or polarization components of the object.
2. Description of the Related Art
There are many applications for which it may be useful to capture multiple images of the same object simultaneously. These images may be filtered in different ways, thus providing different information about the object. For example, in multispectral and hyperspectral systems, different wavelength filters may be used to acquire spectral information, and this information may then be used for spectral analysis or identification of substances or the measurement of molecules or other items labeled with fluorophores.
Acquiring these multiple images can be difficult since most commercially available sensor arrays are designed to capture one image at a time. Traditionally, multiple images were acquired simply by time multiplexing (e.g., capturing images one after another in time) or by using two or more imaging systems or detector arrays in parallel.
For example, spectral imaging applications may use a single image camera in connection with a filter wheel. The filter wheel contains wavelength filters that correspond to the wavelength bands of interest. At any one time, only one of the wavelength filters is positioned in the imaging path and the camera captures the filtered image. The filter wheel rotates in order to switch from one wavelength filter to the next, with the camera capturing images one after another. Thus, the multispectral imaging is implemented in a time multiplexed manner. However, the resulting systems can be large and complicated.
An alternate approach is based on dispersive elements such as prisms or gratings. In this approach, dispersive elements are used to spatially separate different wavelengths. The light is typically dispersed along one dimension of the detector array. The other dimension is used to capture one spatial dimension of the object. However, it is difficult to also capture the second spatial dimension of the object. Sometimes, time multiplexing is introduced to capture the second spatial dimension, for example by scanning.
Yet another approach is to use multiple cameras or imaging systems in parallel. Each camera is fitted with a different spectral filter and the bank of cameras capture filtered images simultaneously. However, this increases the overall cost and complexity since the amount of hardware required is significantly increased. In addition, bulky camera systems may introduce parallax problems.
For some applications, it may be possible to attach filters individually to each sensor element. For example, a conventional RGB imaging device may be based on a detector array where red, green and blue filters are attached to each individual detector. The Bayer pattern is one common pattern for arranging these micro-filters on the detector array. However, one disadvantage of this approach is the increased cost and complexity of manufacturing. Because there is a one-to-one correspondence between micro-filters and detectors, and because the micro-filters are attached to the detectors, the micro-filters are the same size as the detectors, which is small. The many different small micro-filters must then be arranged into an array and aligned with the underlying detectors. This may be difficult, especially if a large number of different types of micro-filters are required. Another disadvantage is the lack of flexibility. Once the micro-filter array is attached to the detector array, it is difficult to change the micro-filter array.
Thus, there is a need for better multi-imaging systems, including multispectral and polarization imaging systems.